An essential feature to obtain a good performance of a bipolar battery of the type as disclosed in the published international patent application WO 2005/048390 is a low internal resistance between the terminals. This is achieved by applying a pressure over a plurality of battery cells using a hydrophobic gasket around the entire periphery of each battery cell to prevent electrolyte bridges to be formed between adjacent battery cells. A drawback with prior art batteries is that the amount of pressure applied over the gasket will also be more or less applied over the stacked battery components of the battery cells. Also, the internal resistance is dependent on the level of contact that exists between the battery components, and thus the magnitude of mechanical load on the battery stack. An increased pressure will result in a lower internal resistance, but there are limits to what degree the pressure may be increased without introducing other undesired effects, such as high mechanical stress on the casing.
Bipolar batteries are well known in the prior art, especially stacked bipolar batteries as is disclosed in U.S. Pat. No. 5,374,490 and EP 1 424 744. However, there is a problem with all disclosed prior art batteries in that the electrodes inside each battery cell will change in thickness during operation and which in turn will create a pressure on the casing. The casing may then crack and the battery malfunction due to stress on the casing and an increased internal resistance.
This problem has been addressed in the published international application WO 2006/104442, assigned to the same applicant, and a casing is proposed that has a mechanically compliant arrangement built into the casing of the battery to reduce the forces on the cell stack caused by changes in cell thickness during operation. However, the pressure applied to the sealing gasket around the periphery of each cell is still dependent of the pressure applied to the stacked internal components to maintain a good contact to obtain a low internal resistance.
Thus there is a need for a battery stack arrangement that will permit the internal resistance to be maintained at an acceptable level during normal operation independently of the pressure needed to create a sealed bipolar battery.